Generally, in the process of manufacturing a semiconductor device, fine patterns are formed using photolithography. A number of transfer masks called photomasks are normally used in forming the fine patterns. This transfer mask is generally a fine pattern formed by a metal thin film or the like and provided on a transparent glass substrate, and photolithography is also used in the fabrication of the transfer mask.
A mask blank having a thin film (e.g., a light shielding film (an opaque film)) for forming a transfer pattern (a mask pattern) on a transparent substrate such as a glass substrate is used in fabricating the transfer mask by photolithography. The fabrication of the transfer mask using the mask blank involves a drawing process of drawing a desired pattern on a resist film formed on the mask blank, a developing process of developing the resist film after drawing to form a desired resist pattern, an etching process of etching the thin film as a mask, and a process of stripping and removing the remaining resist pattern with the resist pattern used as a mask. In the developing process, after the desired pattern is drawn on the resist film formed on the mask blank, a developer is supplied to the resist film to dissolve a portion of the resist film which is soluble in the developer, thereby forming the resist pattern. In the etching step, with the resist pattern used as a mask, an exposed portion of the thin film where the resist pattern is not formed is removed by wet etching or dry etching, thereby forming a desired mask pattern on the transparent substrate. The transfer mask is finished in this way.
A phase shift mask, as well as a binary mask having a light shielding film pattern of a chromium-based material on a conventional transparent substrate, is known as the types of a transfer mask. The phase shift mask is configured to have a phase shift film on a transparent substrate. The phase shift film has a predetermined phase difference, and is made of, for example, a material containing molybdenum silicide compound. Further, a binary mask using a material containing a silicide compound of a metal such as molybdenum for a light shielding film is being used. These binary mask and phase shift mask are generally called a transmissive mask herein, and a binary mask blank and a phase shift mask blank, which are used as a master for a transfer mask, are generally called a transmissive mask blank herein.
In recent years, higher integration of semiconductor devices in the semiconductor industry requires fine patterns beyond the transfer limit of the conventional photolithography using ultraviolet light. To enable formation of such fine patterns, EUV lithography which is an exposure technique using extreme ultraviolet (hereafter referred to as “EUV”) is promising. EUV light refers to light in the waveband of the soft X-ray region or the vacuum ultraviolet region, specifically, light with a wavelength of about 0.2 to 100 nm. A reflective mask has been proposed as a transfer mask for use in the EUV lithography. Such a reflective mask has a multilayer reflective film formed on a substrate to reflect exposure light, and an absorber film patterned on the multilayer reflective film to absorb exposure light.
As described above, as a demand for miniaturization in the lithography process increases, problems in the lithography process are becoming prominent. One of the problems concerns defect information on a mask blank substrate or the like, which are used in the lithography process.
The mask blank substrate is demanded to have a higher flatness from the viewpoints of an improvement on the defect quality needed with the recent miniaturization of patterns and the optical characteristics needed for transfer masks. The conventional surface processing methods for mask blank substrates are described in, for example, Patent Literatures 1 to 3.
Patent Literature 1 describes a glass-substrate polishing method of polishing the surface of a glass substrate essentially comprising SiO2 by using a polishing slurry containing colloidal silica with an average primary particle size of 50 nm or less, acid, and water, and adjusted to have a pH of 0.5 to 4, in such a way that the surface roughness, Rms, as measured with an atomic force microscope becomes not more than 0.15 nm.
Patent Literature 2 describes a polishing agent for the synthetic quartz glass substrate, which contains a suppressive colloidal solution and an acidic amino acid to suppress the formation of defects to be detected by a high-sensitivity defect inspection apparatus on the surface of a synthetic quartz glass substrate.
Patent Literature 3 describes a method of controlling the flatness of a quartz glass substrate by placing the quartz glass substrate in a hydrogen radical etching apparatus, and causing hydrogen radicals to act with the quartz glass substrate, so that the flatness can be controlled in the sub-nanometer order.    Patent Literature 1: JP-A-2006-35413    Patent Literature 2: JP-A-2009-297814    Patent Literature 3: JP-A-2008-94649
With the rapid miniaturization of patterns in lithography using an ArF excimer laser or EUV (Extreme Ultra-Violet), the defect sizes of transmissive masks (also called optical masks), such as a binary mask and a phase shift mask, and an EUV mask is a mask are also becoming smaller. To find such fine defects, the wavelength of the inspection light source used in defect inspection is approaching the light source wavelength of the exposure light.
For example, as a defect inspection apparatus for an optical mask, a mask blank, which is a master thereof, and a substrate, high-sensitivity defect inspection apparatuses with an inspection light source wavelength of 193 nm are becoming popular. As a defect inspection apparatus for an EUV mask, an EUV mask blank, which is a master of an EUV mask, and a substrate, high-sensitivity defect inspection apparatuses with inspection light source wavelengths of 266 nm, 193 nm, and 13.5 nm are becoming popular or have been proposed.
The main surface of a substrate used for the conventional transfer mask is managed by surface roughness represented by Rms (root mean square roughness) and Rmax (maximum height) in the fabrication process. Because the detection sensitivity of the high-sensitivity defect inspection apparatus described above is high, however, many false defects are detected in defect inspection of the main surface of the substrate even when the smoothness in compliance with Rms and Rmax becomes higher from the viewpoint of the improvement on the defect quality, raising a problem such that the defect inspection cannot be performed to the end.
The false defect mentioned herein refers to a tolerable irregularity (unevenness) on the substrate surface, which does not affect pattern transfer, and which is erroneously determined as a defect in inspection with a high-sensitivity defect inspection apparatus. When a lot of such false defects are detected in defect inspection, critical defects that affect pattern transfer may be buried in many false defects, so that the critical defects cannot be discovered. For example, a defect inspection apparatus having an inspection light source wavelength of 266 nm or 193 nm, which is currently becoming popular, cannot inspect the presence or absence of critical defects, because over 100,000 false defects are detected. Overlooking critical defects in defect inspection would cause failures in the later mass production process of semiconductor devices, leading to wasteful labor and economical loss.